The present invention relates to PEM fuel cells, and more particularly to a composite separator plate having oriented fibers to enhance the electrical and thermal conductivity of the fuel cell separator plate, and a method of manufacturing same.
Fuel cells have been proposed as a power source for many applications. One such fuel cell is the proton exchange membrane or PEM fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a membrane electrode assembly or MEA. The MEA is a thin, proton-conductive, polymeric, membrane-electrolyte having an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof. Such MEAs are well known in the art and are described in such as U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia.
In general, MEAs are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION(trademark) available from the E. I. DuPont de Nemeours and Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material such as NAFION(trademark) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is interdisposed between sheets of porous, gas-permeable, conductive material which press against the anode and cathode faces of the MEA and serve as the primary current collectors for the anode and cathode, and the mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, as is well known in the art. This assembly is referred to as the MEA/primary current collector assembly herein.
The MEA/primary current collector assembly is pressed between a pair of non-porous, electrically conductive separator plates which serve as secondary current collectors for collecting the current from the primary current collectors and conducting current between adjacent cells internally of the stack (i.e., in the case of bipolar plates) and at the ends of a cell externally of the stack (i.e., in the case of monopolar plates). The secondary current collecting plate contains a flow field that distributes the gaseous reactants (e.g., H2 and O2/air) over the surfaces of the anode and cathode. These flow fields generally include a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header at one end of the channel and an exhaust header at the other end of the channel.
Conventionally, separator plates are formed of a suitable metal alloy such as stainless steel protected with a corrosion-resistant, conductive coating. Recently, efforts have been directed to the development of a composite separator plate. The design parameters are such composite separator plates require that the materials used have certain electrical and thermal conductivity. In this regard, material suppliers are developing high carbon-loading composite plates consisting of graphite powder in the range of 70% to 90% by volume in a polymer matrix to achieve the requisite conductivity targets. Separator plates of this composition survive the corrosive fuel cell environment and, for the most part, meet cost and conductivity targets. However, due to the high graphite loading and the high specific gravity of graphite, these plates are inherently brittle and dense which yield less than desired volumetric and gravimetric stack power densities. Efforts have been made to reduce the fuel cell stack mass and volume by using thinner plates. Unfortunately, the brittle nature of these plates frequently result in cracking and breaking, especially in the manifold sections of the plate, during part demolding, during adhesive bonding, and during stack assembly operations.
Thus, there is a need to provide a suitable composite material for a fuel cell separator plate and a method of manufacture which overcomes the inherent problems associated with high carbon-loading plates and the inferior properties associated therewith. As such, the use of a low carbon-loading, high polymer-loading plate material is desirable to reduce the brittleness of the separator plate and to meet fuel cell stack mass and volume targets. However, at low carbon concentrations, it is extremely difficult to meet the desired electrical and thermal conductivity targets.
It would be beneficial to include a higher aspect ratio conductive filler to increase conductivities, at the same or lower total volume concentration, by reducing the number and width of polymer insulating gaps between individual conductive particles. Such fibers are known to align in the flow direction (i.e., in the in-plane direction) during the injection and/or compression molding process, generating large electrical and mechanical anisotropies in the final part. Unfortunately, in the case of fuel cell separator plates, fiber alignment is required in the through-plane direction (i.e., through the thickness) to meet through-plane conductivity targets while maintaining a relatively low fiber content for reduced material costs.
Therefore, it is desirable to provide a fuel cell separator plate formed of a robust composite material having adequate electrical and thermal conductivity properties and a method of manufacturing such fuel cell separator plates.
The present invention is directed to a composite separator plate for use in a fuel cell stack of the type having a plurality of flow channels formed therein. The composite material of the separator plate include a polymeric material such as a thermoset or thermoplastic polymer and a fibrous conductive filler having a through-plane orientation which provides a path of conductivity through the separator plate between the top and bottom surface thereof.
The present invention is further directed to a method of manufacture which aligns a fibrous conductive filler in the through-plane direction of a separator plate during injection molding, compression molding or injection compression molding for purposes of increasing the electrical and thermal through-plane conductivity of the separator plate. Through-plane alignment of the fibers enables the conductivity targets to be met at low fiber concentrations, which in turn, helps reduce plate costs, weight, volume and thickness. The design geometry of the mold generates the required flow kinematics (i.e., velocity and velocity gradients) to impart tension and shear forces onto the fiber surface during molding, thereby aligning the fibers in the desired through-plane direction. Specifically, the separator plate is molded with an extra land height which aligns the conductive fibers in a through-plane orientation. After the separator plate is removed from the mold, the extra land height is removed to expose the ends of a portion of the conductive fibers at the land surface.